The present invention generally relates to optics and, more particularly, to systems and methods for mechanically tuning external-cavity lasers.
Tunable external-cavity lasers are commonly used for numerous applications ranging from spectroscopy to telecommunications. For instance, in a Dense Wavelength Division Multiplexing (DWDM) carrier system that uses multiple lasers, each of which is set to a specific wavelength, a tunable external-cavity laser can be maintained as a spare for multiple ones of the lasers. In such an application, an inventory of spare lasers that are wavelength specific need not be maintained for the DWDM system.
The basic structure of a conventional tunable external-cavity laser primarily includes an anti-reflection (AR) coated optical gain medium placed in an external cavity with a diffraction grating. Interaction between the gain medium and the grating creates feedback of a tunable wavelength. In particular, the diffraction grating within the external cavity forces the gain medium to operate in a single longitudinal mode by creating a wavelength-dependent loss within the external cavity and, thus, enables one or more wavelengths to be selected or tuned. AR coating is applied on the front facet of the gain medium in order to suppress self-lasing between the front and back facets. The AR coating, therefore, prevents the laser from operating in a mode determined by the facets and, thus, the laser operates in a mode determined by the external diffraction grating. On the back facet, a highly reflective coating is used to reflect the light through the front facet, through a collimating lens, and onto the grating, where the first order diffraction beam is directed back onto the optical gain medium. The beam is then amplified and coupled out of the laser as the zero order diffraction beam. Alternatively, the light can be coupled out of the cavity through a partially-transmitting back facet.
Tunable external-cavity lasers typically are considered too large in size and too mechanically sensitive for use in optical networks. As demonstrated in U.S. Pat. No. 5,802,085, issued on Sep. 1, 1998, and shown in FIG. 1, a typical tuning mechanism includes a stepper motor 110, a support 120 with an attached frequency selective reflective element 130, such as a mirror or reflective diffraction grating, and a pushrod 140. In this general type of device, the pushrod 140 converts the rotary motion of the stepper motor 110 into linear motion. Accordingly, the pushrod 140, via the stepper motor 110, drives the support 120 and frequency selective reflective element 130 about a pivot axis P to adjust the orientation of the frequency selective reflective element 130 with respect to the propagation axis of the light emitted from the optical gain medium 105.
As a result of the support being held in place only by a pushrod and possibly a spring, as demonstrated in U.S. Pat. No. 5,319,668, issued on Jun. 7, 1994, the support in these types of tuning mechanisms often moves, e.g., shakes out of position, when subjected to vibration. Such vibration is present in a forced-air cooled equipment rack, for example. Moreover, the rotation angle of the reflective element is not directly proportional to the rotation angle represented by the motor step count, since the pushrod converts the motor""s rotary motion to a linear motion. This can make the laser difficult to tune and control. Both U.S. Pat. Nos. 5,802,085 and 5,319,668 are incorporated herein by reference.
Therefore, there is a need for improved systems and methods that address these and/or other perceived shortcomings of the prior art. For example, there is a need in the industry for an improved system and method that provide compact, efficient, and robust mechanical tuning of an external-cavity laser.
The present invention involves external-cavity laser tuning mechanisms. These tuning mechanisms use opposing drive surfaces to engage the supports upon which reflective elements are attached. This fixes the position of such a reflective element. Repositioning of the reflective element for tuning is accomplished by a motor, the rotation of which is directly proportional to the rotation of the reflective element about its pivot.
In this regard, the present invention provides systems and methods for improved mechanical tuning of an external-cavity laser. Briefly described, one such system includes a motor and first and second opposing surfaces that are displaceable by the motor. The opposing surfaces operatively engage a drive segment of the support and rotate the drive segment about a pivot point on the support. Between the drive segment and the pivot point of the support, a reflective element is attached. An optical gain medium optically communicates with the reflective element.